1. Field of the Invention
The present invention relates to a scroll type compressor, and more particularly, to a scroll type compressor with a variable displacement mechanism.
2. Description of the Prior Art
A compressor in an automobile air conditioning system is driven by the automobile engine through an electromagnetic clutch. Unless the compressor has a variable displacement mechanism, when the engine rotates at a high rate, the compressor will be driven at a high rate as well and the operating capacity of the compressor may be larger than necessary. Therefore, in order to ensure proper functioning of the compressor, the electromagnetic clutch must be turned on and off frequently. This frequent control of the electromagnetic clutch causes a large change in the load on the engine, thereby reducing the speed and acceleration performance of the automobile.
One solution to above problem is to provide a scroll type compressor with a variable displacement mechanism for varying the compression ratio. A scroll type compressor with a variable displacement mechanism is disclosed in U.S. Pat. No. 4,717,314 to Sato et al., which is hereby incorporated by reference. The variable displacement mechanism includes a control device which controls an opening and closing of a communication path between the suction chamber and a pair of intermediately located sealed spaces defined by the spiral elements. The control device includes a cylinder, a part of which defines the communication path, and a piston member which is slidably disposed within the cylinder. The control device further includes an electromagnetic valve which is magnetized and demagnetized in response to an external ON-OFF signal to introduce discharge pressure to an upper surface of the piston member. By selectively introducing discharge pressure to the upper surface of the piston member, the piston member slides within the cylinder to control the opening and closing of the communication path.
However, the variable displacement mechanism of U.S. Pat. No. 4,717,314, in order to generate the external ON-OFF signal, requires an electromagnetic valve and a device for processing a signal representing an operational condition of the automobile air conditioning system (e.g., temperature of the air leaving the evaporator). The requirement of an electromagnetic valve and the associated signal processing device increases the number of component parts of the variable displacement mechanism. Therefore, the manufacturing cost of the compressor is increased.
Alternatively, a variable displacement mechanism for a scroll type compressor can operate without the provision of an electromagnetic valve. For example, U.S. Pat. No. 4,940,395, which is hereby incorporated by reference discloses a variable displacement mechanism for a scroll type compressor which relies on the piston's reciprocation to open and close a communication path between intermediately located fluid pockets and the suction chamber. The '395 piston is hollowed out and includes a suction responsive bellows valve placed therein. When the pressure in the bellows overcomes the suction pressure in the interior of the piston, the bellows expands, and a needle-ball type valve sealing the discharge pressure at the top of the piston is unseated. Consequently, the discharge pressure acting on the top of the piston is exhausted to the suction chamber, and the piston, biased by the restoring force of coil spring, reciprocates to open the communication path between the intermediate fluid pockets and the suction chamber.
While disposing with the need for an electromagnetic valve, the '395 variable displacement mechanism still requires a piston whose reciprocal movement is dependent upon a bellows disposed therein. Therefore, the '395 piston valve member is somewhat complicated.
Other scroll type compressors having variable displacement mechanisms include U.S. Pat. Nos. 4,505,651, 4,642,034, 4,673,340, 4,744,733, and 4,904,164, which are all hereby incorporated by reference.